Mercury 2013 3 Cyl. 4 stroke Vibration

[yarcraft91]

Originally Posted by MarkG

I agree with this statement the most. Plus the small mass of a 40hp the motor itself, seems to play into this to some extent,compared to larger motors. It could also be the idle is set just a bit too low. On my carbed F40 Yamaha, I smoothed it out considerably , just by bumping up the idle a few RPMs,not even 100. Easily done with the idle stop screw.
Since your Merc is newer, and likely EFI, Not sure there is any mechanical adjustment to do this unless your motor has a variable idle/trolling control feature of some kind. But A dealer might be able to set the idle via the computer. Of course if you troll, the downside of bumping it up,is that it will result in some increase of trolling speed.

Hard to say. My Merc 50 EFI is a 2006 model and there is a screw I can turn to adjust idle speed. It takes a special tool to turn that screw, apparently it's not intended for adjustment by the customer. That screw may have been eliminated in later models.

[Jimmy Jig]

The 40 hp has no screws, or something to grind off to slow it down.

[Bobby Winds]

Originally Posted by jrdonicht

Your problem may be completely normal with that 40hp. Smaller outboards 2 cylinder vs 3 cylinder vs larger 4 cylinder. You can't compare a three cylinder motor to a four cylinder motor when it comes to vibration. I had a 40 honda[3 cylinder] that would shake way more than my Yamy f75. Its just the nature of the smaller fourstrokes. Of course I am not there with yours. Best to compare with another one if you can.

I did have a 1985 50 hp Johnson VRO that trolled down real nice for 30 years and was out today in my buddy's boat that has the same old 50 hp duck hunting and it too still runs smooth even at idle speeds.

It's something with 3 cylinder motors I "guess'....

[REW]

A very good white paper on engine balance for 1-10 cylinder engines.

https://en.wikipedia.org/wiki/Engine_balance

[staylor]

As others have posted, 3 cyl motors vibrate more than 4 cyl, but I would not give up just yet. For a start, try dropping down to 87 octane fuel- which will burn faster than the 91 you are running- and this may run smoother at low rpm. Next, take a really good look at the throttle linkage and see what is stopping the throttle when you get back to idle- the stop may be at the throttle butterfly or shutter, or it could be somewhere in the various linkage arms. In any event, the typical Merc setting for the factory set idle stops is such that when the throttle/shifter is at idle in neutral, a piece of notebook paper inserted between the stop and the linkage should pull out with a light drag but not tear. If your throttle cable adjustment is not correct, you may find the motor is not coming down to the proper stop position and this may be messing up the throttle/ignition timing curve since the ECU cannot find the proper zero setting. On my 2014 Merc 150 4S my throttle cables must have stretched in the first year I ran it and I needed to adjust the throttle cables to get the linkage to bottom on the stop properly. I knew something was wrong because the motor's idle was rougher than I remembered it to be when starting the second season of use. I'll assume you've also pulled the spark plugs and verified they are gapped properly and free of carbon deposits. Do not assume the factory set the gaps properly- they tend to set them around .002 to .004 under spec so that after X number of hours the gaps will wear into spec. For a motor that's going to troll a lot- the gaps should be set exactly to spec or maybe even .001 wider for best idling.

Lastly, take a good look at the external rubber engine vibration mounts that the motor is rocking against at idle. Many Merc mounts are such that the rubber in them can bind up a bit as they age- and often these can be greased by squishing grease down into the area where the rubber strokes at idle. Lubing these with some grease may loosen them up a bit to allow less shaking at idle.
Doug

[yarcraft91]

Originally Posted by Bob/MN

The motor is a 2013 hold over with not more than 50 total hour on it. I always use 91 octane fuel. The dealer checked it out and winterized the motor. I will try the seafoam thick in the spring.

I agree with staylor- run your outboard on 87 octane. I've run my 2006 50 EFI 4-stroke on unleaded E10 87 octane (always with a fuel stabilizer like blue Stabil added) since it was new and haven't had a problem with it yet. These outboards are designed for 87 octane fuel.

[Bob/MN]

I motor is in storage as we are at the beginning of the hard water season. I will do the checks in the spring.
Thanks for the good advice.